Saturday, September 21, 2013

I see with light but I don't see light

I've been teaching light as wave (and then light as a particle) for years, but only last year did the words "We don't actually see light" (as a wave) come out of my mouth.  I then paused and said "but of course seeing light is actually what seeing is".  The class and I looked at each other like we were all deer caught in the headlights.  I think we were all on the same page at that moment in the classroom, but it is a bit linguistically problematic, isn't it?

Every year, at least one student will ask a question that reveals that age-old misconception that comes from our describing light as a transverse wave:  they are trying to picture photons going in a wave-like path on their way from source to sink.  Adding to, not helping this easy-to-make mistake is the classic picture of light as an propagating electric & magnetic field:


Look - there's stuff going up & down  (& sideways!).  This is what we show the students to justify calling it a transverse wave.  But, nothing is actually "going" up or down.  Those arrows represent field intensities getting bigger and smaller (and changing direction) along the central arrow.  This picture assume a relatively sophisticated understanding of vector fields which first year physics student don't have.

A picture is worth a thousand words, but what if 500 hundred of those words are the wrong words?

1 comment:

  1. Valid point Rideout. Much of the time teachers assume students understand things prior to class lectures; this is a clear misconception. (E.g.) I wonder if Kamal knows what a vector is on this very day. -Matt Goddard

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